The field of art to which this invention pertains is water remoistenable adhesive compositions.
Remoistenable adhesives are dry adhesives composition which, when moistened with water, develop an adhesive tackiness. Such adhesives are used, for example, on postage stamps, labels and envelope flaps.
Remoistenable adhesives have been prepared by dissolving a water soluble resin in a suitable solvent and then applying the solution to the paper. Adhesive coated paper produced in this manner is troublesome in practical use because of the tendency of the coated sheet to curl or roll up. This effect is due to the fact that the paper backing and the adhesive layer respond differently to changes in atmospheric moisture content. Both the paper backing and the adhesive layer expand as they absorb moisture from the atmosphere and contract as they release moisture to the atmosphere. However, normally, this behavior is much more pronounced in the adhesive layer which expands or contracts to a greater degree than the paper backing. If such a sheet of adhesive coated paper is subjected to an atmosphere of high moisture content, the adhesive layer will expand more than the paper backing causing the sheet to curl away from the adhesive side. If the sheet is subjected to an atmosphere of low moisture content, the adhesive layer will contract more than the paper backing causing the paper to curl toward the adhesive side.
Manufacturers have attempted to overcome this curling effect by passing the coated sheet over a series of bars set at various angles which cause the adhesive layer to be broken into small sections. Theoretically, each small section thus produced reacts to variation in moisture content of the atmosphere independent of all other sections thus lessening the tendency of the sheet to curl. In actual practice this technique is only partially effective since that portion of the adhesive layer in contact with the paper surface is reinforced by paper fibers and is particularly resistant to rupture. Even though the surface of the adhesive layer is broken, that portion of the film in contact with the paper remains continuous and reacts to variations in atmospheric moisture content with sufficient force to curl.
U.S. Pat. No. 2,793,966 describes a remoistenable adhesive composition which when used on paper overcomes this curling tendency. The adhesive composition is made by dispersing finely divided dextrin in an organic solvent solution of a film forming resin. When this composition is applied to paper and the solvent is evaporated off, a solid remoistenable film is produced which to the naked eye appears continuous and homogeneous. In actuality, the film is composed of an aggregation of minute discrete particles of solid dextrin which are adhered to the paper by the film forming polymer. Paper coated with this adhesive has been found to be substantially non-curling.
U.S. Pat. No. 3,202,539 describes non-curling gummed papers made by blending a water soluble powder glue in a dry powder water insoluble binder material. This mixture is applied to the substrate and the binder material is fused to a continuous film with heat.
U.S. Pat. No. 2,917,396 describes a water remoistenable material which is made of a dispersion of an aqueous solution of a water activatable gum dispersed in a water immiscible solvent solution of a water soluble resinous binder.
U.S. Pat. No. 2,978,343 describes a remoistenable adhesive composition made from a solution of a water soluble adhesive in a water immiscible organic solvent, and a second water soluble adhesive which is suspended in the solution and which is insoluble in the particular organic solvent used, plus a critical amount of water.
Pre-moistenable adhesives are made according to U.S. Pat. No. 3,200,094, by dispersing dextrin in a water/organic solvent solution of polyvinyl alcohol.
U.S. Pat. No. 3,425,968 describes water remoistenable adhesives made by dissolving a glue, such as dextrin, in water and mixing this solution with a solution of a water insoluble binder in a organic solvent.
As can be seen, various attempts have been made to solve the curl problems of water remoistenable adhesives. Some of these adhesives are totally organic and contain organic solvents which cause environmental pollution problems. Others are unstable in the package. The dispersed glues have a tendency to settle out and cake which cause problems in use. The water present in some of these compositions will dissolve the dispersed water soluble glue with time, resulting in adhesives which when applied cause curling of the paper substrate.